Description of the Prior art
Fertilizers and phosphoric acid are prepared from phosphate rock. Although a minor fraction of the phosphates is of magmatic origin (apatite), the major portion of the phosphate deposits on the earth are found as sedimentary phosphates (phosphate rock).
Apatites are almost devoid of radioactivity, while phosphate rock possesses some, albeit weak, radioactivity, which varies with the type of phosphate rock. This radioactivity is attributable to the uranium content of the phosphate rock and to the radium which is present due to the decay of the uranium. In the conventional method of preparing phosphoric acid by the decomposition of phosphate rock with sulfuric acid, the radium present in the phosphate is almost completely removed in the process together with the calcium sulfate as the very slightly soluble radium sulfate RaSO.sub.4, and remains in the sulfate precipitate, while the uranium remains dissolved in the phosphoric acid which is principally used for fertilizer.
A small amount of radioactivity in a fertilizer can cause practically no radiation injury since it is widely distributed when the fertilizer is spread on the land. Furthermore, the radium remains fixed in the soil as a consequence of the extraordinarily low solubility of RaSO.sub.4, and thus is not taken up by the plants. Accordingly, health problems never arise from fertilizers and their use.
In the production of gypsum, the radioactivity caused by the presence of RaSO.sub.4 causes no trouble provided that the gypsum is disposed of in the ocean or in a dump, as is generally the case. Furthermore, the radioactivity causes no trouble when the gypsum from phosphoric acid production is used as a retardant in cement, since generally only 3-5% CaSO.sub.4 is added to the cement. As a consequence of this dilution, the radioactivity of the cement can in general be neglected. However, the radioactivity of the gypsum from phosphoric acid production can be a problem whenever the gypsum is used directly for building purposes, e.g. for production of gypsum plasterboard, gypsum wallboard, or gypsum stone. In gypsum plaster, the radioactivity is not so important as in the above-mentioned building elements, because the quantity of gypsum added is smaller.
In order to ensure a multi-faceted application of the gypsum from phosphoric acid production, it is desirable to reduce the radioactivity. In the report commissioned by the German Federal Department of the Interior, "Exposure to external radiation in the German Federal republic by naturally radioactive materials in the open and in dwellings, taking into account the effect of building materials", the radioactivity of building materials, among others, is given.
According to this source, most of the synthetic gypsum derived from phosphate rock has a radioactivity between 10 and 25 nCi/kg, while some is over 25 nCi/kg. On page 28 of the report, the following limits for building materials are given:
radium and thorium content; PA1 &lt;10 nCi/kg--unlimited application PA1 10-25 nCi/kg--must be disclosed PA1 &gt;25 nCi/kg--cannot be used.
This report also establishes as a goal the development of a process for preparing building gypsum whereby a gypsum from production of phosphoric acid having a radioactivity of less than 10 nCi/kg can be prepared even from phosphate rock containing relatively large amounts of uranium.